Tire comprising a tread formed by multiple elastomer blends

ABSTRACT

A tire of which the tread contains at least two layers of blended elastomeric compounds that are radially superposed and has a voids ratio that is lower in the central part than at the axially outer parts thereof. A first layer of blended elastomeric compounds of the tread is made up of a first blended elastomeric compound forming a central part, and of two outer parts formed of a second blended compound, the first blended compound having a Z-value higher than 55, the first blended compound and the second blended compound having a tan(δ)max value lower than 0.110, the modulus G* of the second blended elastomeric compound having a value at least 10% higher than that of the modulus G* of the first blended elastomeric compound, a second layer of the tread, radially the outermost, being made up of a third blended compound of which the modulus G* has a value at least 10% higher than that of the modulus G* of the first blended elastomeric compound and the elongation at break of the third blended compound having a value at least 10% higher than those of the elongations at break of the first and second blended compounds.

This application is a 371 national phase entry of PCT/EP2013/063535, filed 27 Jun. 2013, which claims benefit of French Patent Application No. 1256486, filed 5 Jul. 2012, the entire contents of which is incorporated herein by reference for all purposes.

BACKGROUND

1. Field

The disclosure relates to a tire with a radial carcass reinforcement intended to be fitted to a heavy vehicle such as a transport vehicle or an “industrial” vehicle. It is notably a tire that has an axial width greater than 37 inches.

Although not limited to this type of application, the disclosure will be more particularly described with reference to a vehicle of the “dumper” type weighing in excess of 300 tonnes, fitted with tires of a diameter greater than three meters fifty with an axial width greater than 37 inches.

2. Description of Related Art

Such a tire, intended in general to carry heavy loads, comprises a radial carcass reinforcement and a crown reinforcement made up of at least two working crown plies formed of inextensible reinforcing elements which are crossed from one ply to the next and make equal or unequal angles of between 10 and 45° with the circumferential direction.

As far as the usual design of tires for industrial vehicles is concerned, the radial carcass reinforcement, which is anchored in each bead, is made up of at least one layer of metal reinforcing elements, the said elements being substantially parallel to one another within the layer. The carcass reinforcement is usually surmounted by a crown reinforcement made up of at least two working crown layers of metal reinforcing elements, these however being crossed from one layer to the next and making angles of between 10 and 65° with the circumferential direction. Between the carcass reinforcement and the working crown layers there are usually two layers of reinforcing elements, crossed one ply to the next and at angles smaller than 12°; the width of these layers of reinforcing elements is usually less than those of the working layers. Radially on the outside of the working layers there are also protective layers the reinforcing elements of which are at angles of between 10 and 65°.

Radially on the outside of the crown reinforcement is the tread usually made up of polymeric materials intended to come into contact with the ground in the contact patch in which the tire is in contact with the ground.

Cords are said to be inextensible when the said cords exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.

Cords are said to be elastic when the said cords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.

The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction of running of the tire.

The axis of rotation of the tire is the axis about which it turns in normal use.

A radial or meridian plane is a plane containing the axis of rotation of the tire.

The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured in the axial direction. The expression “axially on the inside of or, respectively, axially on the outside of” means “of which the axial distance, measured from the equatorial plane, is less than or, respectively, greater than”.

The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto. A radial distance is measured in the radial direction. The expression “radially on the inside of or, respectively, radially on the outside of” means “of which the radial distance, measured from the axis of rotation of the tire, is less than or, respectively, greater than”.

In the case of vehicles, notably those intended for use in mines or quarries for transporting loads, the difficulties regarding access routes and profitability requirements lead the manufacturers of these vehicles to increase their load-carrying capability. It then follows that the vehicles become increasingly large and, therefore, increasingly heavy themselves so that they can transport an increasingly heavy load. At the present time, these vehicles may reach weights of several hundreds of tonnes and so may the weight of the load to be transported; the overall weight may therefore be as much as 600 tonnes.

Increasing demands are therefore placed on the tires. The tires have simultaneously to exhibit good performance in terms of wear, be capable of transporting the necessary torque and of withstanding the attack notably from rocks encountered on the tracks.

Document FR 1445678 thus proposes choosing different materials in the axial direction for forming the tread. The central part may be made up of a more wear-resistant material than the lateral parts.

Wear-resistant materials usually carry a penalty in terms of hysteresis properties. Thus it is also known practice for the tread of a tire to be made of a radial superposition of two different materials in order to reach a compromise between wear properties and hysteresis properties that is satisfactory for the envisaged applications.

Such tires are, for example, described in document U.S. Pat. No. 6,247,512. That document describes the superposition of two layers of materials in order to form the tread, the outer material coming into contact with the ground notably exhibiting better performance in terms of wear whereas the inner material has hysteresis properties that make it possible to limit increases in tire temperature in the crown region.

With this type of tire, the patterns present on the tread may also vary in the axial direction; thus it is known practice to have a lower voids ratio in the central part in order to transmit torque, and also avoid attack in the central part where rocks are more difficult to remove.

The voids ratio of the tread is defined according to the invention by the ratio of the surface area of sipes or grooves to the total area of the tread. The voids ratio of a part of the tread is thus defined according to the invention by the ratio of the surface area of sipes or grooves present in the said part of the tread, to the total surface area of the said part of the tread.

SUMMARY

The inventors set themselves the task of providing tires with an even better compromise between the various performance aspects of wear, resistance to attack, and endurance.

According to an embodiment of the invention, this object has been achieved using a tire with a radial carcass reinforcement, comprising a crown reinforcement, itself capped radially by a tread connected to two beads by two sidewalls, the said tread comprising at least two layers of blended elastomeric compounds that are radially superposed and have a voids ratio that is lower in the central part than at the axially outer parts, a first layer of blended elastomeric compounds of the tread being made up of a first filled blended elastomeric compound forming a part extending at least into the region of the equatorial plane and of at least two axially outer parts formed of a second blended elastomeric compound, the first blended elastomeric compound having a macro dispersion Z-value higher than 55, the first blended compound and the second blended compound having a maximum tan(δ) value, denoted tan(δ)max, lower than 0.110, the complex dynamic shear modulus G* 1% at 100° C. of the second blended elastomeric compound having a value at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the first blended elastomeric compound, a second layer of the tread, radially on the outside of the said first layer, being made up of a third blended elastomeric compound of which the complex dynamic shear modulus G* 1% at 100° C. has a value at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the first blended elastomeric compound, and the elongation at break of the third blended elastomeric compound in a tearability test at 100° C. having a value at least 10% higher than those of the elongations at break of the first and second blended elastomeric compounds in a tearability test at 100° C.

A macro dispersion Z-value higher than 55 for a filled blended elastomeric compound means that the filler is dispersed in the elastomeric matrix of the composition with a dispersion Z-value of 55 or higher.

In the present description, the dispersion of filler in an elastomeric matrix is characterized by the Z-value which is measured, after crosslinking, using the method described by S. Otto et al in Kautschuk Gummi Kunststoffe, 58 Jahrgang, NR 7-8/2005, which is consistent with ISO standard 11345.

The Z-value calculation is based on the percentage of surface area in which the filler is not dispersed (“non-dispersed % area”) as measured by the “disperGRADER+” device supplied with its operating procedure and “disperDATA” exploitation software by the company Dynisco, using the equation: Z=100−(non-dispersed % area)/0.35

The non-dispersed percentage area is itself measured using a camera that looks at the surface of the specimen under light incident at an angle of 30°. Light-coloured points are associated with filler and agglomerations, while dark points are associated with the rubber matrix; a digital processing operation converts the image into a black-and-white image and allows the percentage of non-dispersed area to be determined, in the way described by S. Otto in the abovementioned document.

The higher the Z-value, the better the dispersion of the filler in the rubber matrix (a Z-value of 100 corresponding to a perfect dispersion and a Z-value of 0 to a mediocre dispersion). A Z-value of 55 or higher is considered to correspond to a satisfactory dispersion of the filler in the elastomeric matrix. In particular, in the case of a reinforcing filler made up of a cut of carbon black and of a white filler, a Z-value of 55 or higher corresponds to a good dispersion of the filler in the elastomeric matrix.

The blended elastomeric compounds of which the tread is made up are prepared using known methods.

In order to achieve a macro dispersion Z-value higher than 55, the blended elastomeric compound that makes up the radially outer part may advantageously be prepared by forming a master batch of diene elastomer and reinforcing filler.

Within the meaning of the disclosure a “master batch” means an elastomer-based composite into which a filler has been introduced.

There are various ways of obtaining a master batch of diene elastomer and reinforcing filler. In particular, one type of solution is, in order to improve the dispersion of the filler in the elastomer matrix, to mix the elastomer and the filler in the “liquid” phase. To do that, use is made of an elastomer in the form of a latex which takes the form of particles of elastomer dispersed in water, and of an aqueous dispersion of the filler, namely a filler dispersed in water and commonly referred to as a “slurry”.

Thus, according to one of the alternative forms of the invention, the master batch is obtained by liquid-phase mixing from a diene elastomer latex containing natural rubber and an aqueous dispersion of a filler containing carbon black.

More preferably still, the master batch according to an embodiment of the invention is obtained using the following process steps, that make it possible to obtain very good dispersion of the filler in the elastomer matrix:

-   -   supplying a continuous first stream of a diene elastomer latex         to a mixing zone of a coagulation reactor defining an elongate         coagulation zone extending between the mixing zone and an         outlet,     -   supplying the said mixing zone of the coagulation reactor with a         second continuous stream of a fluid containing a filler under         pressure in order to form a mixture with the elastomer latex by         mixing the first fluid and the second fluid in the mixing zone         vigorously enough to cause the elastomer latex to coagulate with         the filler before the outlet, the said mixture flowing as a         continuous stream towards the outlet zone and the said filler         being capable of coagulating the elastomer latex,     -   collecting at the outlet to the reactor the coagulum obtained         previously in the form of a continuous stream and drying it in         order to recover the master batch.

A method of preparing a master batch in the liquid phase is described for example in document WO 97/36724.

The loss factor tan(δ) is a dynamic property of the layer of blended rubber compound. It is measured using a viscoanalyser (Metravib VA4000) in accordance with standard ASTM D 5992-96. The response of a sample of vulcanized composition (cylindrical test specimen 4 mm thick and 400 mm² in cross section), subjected to simple alternating sinusoidal shear stressing at a frequency of 10 Hz, and a temperature of 100° C. is recorded. The amplitude of deformation sweeps from 0.1 to 50% (in the outbound cycle) and then from 50% to 1% (in the return cycle). The results exploited are the complex dynamic shear modulus (G*) and the loss factor tan(δ). In the return cycle, the maximum observed value of tan(δ), denoted tan(δ)_(max) is indicated.

The rolling resistance is the resistance that arises when the tire is rolling and indicates the rising temperature of the said tire. It is thus represented by hysteresis losses associated with the deformation of the tire over one revolution. The tan(δ) values for the materials used are measured at 10 Hz between 30 and 100° C. in order to incorporate the effect of the various frequencies of deformation caused by the turning of the tire. The value of tan(δ) at 100° C. thus corresponds to an indication of the rolling resistance of the tire during running.

The various measurements are taken on new tires that have not yet been run at all.

According to a preferred embodiment of the invention, the voids ratio of the central part of the tread is between 2 and 15% and, for preference, less than 10%.

For preference also, the voids ratio of the axially outer parts of the tread is between 20 and 40% and preferably less than 30%.

The inventors have been able to demonstrate that the tread being made up of at least two layers that are radially superposed, the radially inner first layer being formed of the combination of a first filled blended elastomeric compound having a macro dispersion Z-value higher than 55, by way of material for the central part of the layer of the tread coinciding at least in part with the region with the lowest voids ratio and of a second blended elastomeric compound having a value for the complex dynamic shear modulus G* 1% at 100° C. that is at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the first blended elastomeric compound, by way of material for the axially outer parts of the tread coinciding at least in part with the region of highest voids ratio and of a second layer coming into contact with the ground being made up of a third blended elastomeric compound of which the complex dynamic shear modulus G* 1% at 100° C. has a value at least 10% higher than that of the first blended elastomeric compound and the elongation at break of the third blended elastomeric compound in a tearability test at 100° C. having a value at least 10% higher than those of the elongations at break of the first and second blended elastomeric compounds in a tearability test at 100° C. leads to a compromise between wear resistance and resistance to attack properties while at the same time favoring the transmission of torque.

Specifically, as mentioned previously, the voids ratio in the middle of the tread is lower by comparison with the axially outer parts. This reduction in the voids ratio in the central part of the tread makes it possible to safeguard the crown reinforcement from attack, it notably being a tricky business to remove stones from this central part of the tread.

Choosing a radially outermost second layer made up of a third blended elastomeric compound of which the complex dynamic shear modulus G* 1% at 100° C. has a value at least 10% higher than that of the first blended elastomeric compound of the radially inner layer and of which the elongation at break of the third blended elastomeric compound in a tearability test at 100° C. has a value at least 10% higher than those of the elongations at break of the first and second blended elastomeric compounds of the radially inner layer in a tearability test at 100° C. contributes to better resistance of the tire to attack across the entire width of its tread.

Indeed it would seem that the values for the complex dynamic shear modulus G* 1% at 100° C. of the third blended elastomeric compound make it possible to limit the starts of cracks that may appear as a result of stones becoming jammed in the tread pattern. Moreover, it would seem that the values of elongation at break of the third blended elastomeric compound make it possible to limit the spread of started cracks that may appear as a result of the presence of the stones. Such properties of the third blended elastomeric compound in fact give it cohesion properties that favor resistance to attack.

Next, choosing the first filled blended elastomeric compound with a macro dispersion Z-value higher than 55 as the material for the central part of the tread leads to wearing properties, in particular after the wearing away of the radially outermost second layer, that are particularly advantageous in terms of the use of such tires, the inventors having been able to demonstrate that the most pronounced wear occurs in the central part of the tread, which is the part most heavily loaded when transmitting torque.

Choosing the second blended elastomeric compound that has a value for the complex dynamic shear modulus G* 1% at 100° C. that is at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the first blended elastomeric compound as the material for the axially outer parts of the tread also contributes, in the same way as stated previously in respect of the third blended elastomeric compound, to better resistance of the tires to attack notably in those parts of the tread that have the highest voids ratio.

Finally, choosing the first and second blended elastomeric compounds as the material for the radially inner first layer of the tread which also has a tan(δ)max value lower than 0.110 leads to hysteresis properties that favor lower heating of the tire and thus better tire endurance.

Advantageously according to an embodiment of the invention, the complex dynamic shear modulus G* 1% at 100° C. of the first blended elastomeric compound forming a part that extends at least into the region of the equatorial plane of the first layer of blended elastomeric compounds is higher than 1.4 and is preferably 2 or lower.

Advantageously also according to an embodiment of the invention, the complex dynamic shear modulus G* 1% at 100° C. of the second blended elastomeric compound forming the said at least two axially outer parts of the first layer of blended elastomeric compounds, is higher than 1.6 and is preferably 2.4 or lower.

Advantageously also according to an embodiment of the invention, the complex dynamic shear modulus G* 1% at 100° C. of the third blended elastomeric compound forming the radially outer second layer is higher than 1.6 and is preferably 2.4 or lower.

Advantageously according to an embodiment of the invention, the ratio MSA300/MSA100 of the first blended elastomeric compound has a value at least 5% higher than that of the ratio MSA300/MSA100 of the second blended elastomeric compound.

Advantageously according to an embodiment of the invention, the ratio MSA300/MSA100 of the first blended elastomeric compound has a value which differs by at most 5% from that of the ratio MSA300/MSA100 of the third blended elastomeric compound of the said second layer.

Tensile testing makes it possible to determine the elastic stresses and properties at break. It is carried out in accordance with French standard NF T 46-002 of September 1988. Processing the results of the tensile testing makes it possible to plot the curve of modulus as a function of elongation. The modulus used here is the nominal (or apparent) secant modulus measured in first elongation, calculated with respect to the initial cross section of the test specimen. The nominal secant modulus's (or apparent stresses, in MPa) are measured in first elongation at 100% and 300% elongation and referred to respectively as MSA 100 and MSA 300. The stresses at break (in MPa) and the elongations at break (in %) are measured at 23° C.±2° C. and at 100° C.±2° C. in accordance with standard NF T 46-002.

According to a preferred embodiment of the invention, the first blended elastomeric compound forming a part that extends at least into the region of the equatorial plane of the first layer of blended elastomeric compounds contains, by way of reinforcing filler, at least a carbon black with a BET specific surface area higher than 100 m²/g, used in a proportion of between 10 and 70 phr.

For preference also, the reinforcing filler of the first blended elastomeric compound forming a part that extends at least into the region of the equatorial plane of the first layer of blended elastomeric compounds contains by way of reinforcing filler a cut of carbon black as described hereinabove and of a white filler, the overall level of filler being between 10 and 90 phr and the ratio of carbon black to white filler being higher than 2.7.

The selection of the fillers as described hereinabove makes it possible further to improve the wear resistance properties of the first blended elastomeric compound of the first layer of blended elastomeric compounds.

The thickness, measured in the radial direction at the end of what in a meridian section of the tire is the radially outermost working layer, of the second layer of blended elastomeric compound is preferably greater than 15% and, more preferably still, greater than 100% of the thickness, measured in the radial direction at the end of what in a meridian section of the tire is the radially outermost working layer, of the first layer of blended elastomeric compound.

One advantageous alternative form of the invention also provides for the presence of an additional layer of a blended elastomeric compound in a radially innermost position in the tread and therefore in contact with the crown reinforcement of the tire, having a maximum tan(δ) value, denoted tan(δ)max, lower than 0.80.

The presence of this additional layer radially furthest towards the inside of the tread may make it possible to reduce still further the increases in tread temperature and therefore favor the endurance of the tire.

The presence of this additional layer radially furthest towards the inside of the tread may also make it possible to reduce the operating temperature of the tire in the crown region in order to make it possible to lower the rolling resistance of the tire.

The thickness of this additional layer of blended elastomeric compound, measured in the radial direction at the end of the working layer that is radially outermost in a meridian section of the tire, is advantageously between 15% and 25% of the thickness of the complete tread measured, in the radial direction at the end of the working layer that is radially outermost in a meridian section of the tire.

According to embodiments of the invention, the aforementioned thickness measurements are taken from tires in the new state, which have not been run and therefore exhibit absolutely no tread wear.

BRIEF DESCRIPTION OF DRAWINGS

Further details and advantageous features of the invention will become apparent hereinafter from the description of some embodiments of the invention given with reference to FIGS. 1 and 2 which depict:

FIG. 1, a diagrammatic meridian section of a tire according to embodiments of the invention,

FIG. 2, a diagrammatic meridian view of the tread of the tire of FIG. 1.

For ease of understanding, the figures have not been drawn to scale.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 schematically depicts a tire 1 intended to be used on vehicles of the dumper type. It comprises a radial carcass reinforcement 2 anchored in two beads 3 around bead wires 4. The carcass reinforcement 2 is formed of a layer of metal cords. The carcass reinforcement 2 is hooped by a crown reinforcement 5, itself capped by a tread 6. The tread 6 is, according to an embodiment of the invention, made up of a central part 7 extending at least into the region of the equatorial plane XX′, and of two axially outer parts 8, 9.

According to an embodiment of the invention, the central part 7 of the tread 6 has a voids ratio (the tread patterns are not depicted in the figures) which is lower than that of the axially outer parts 8 and 9.

FIG. 2 very schematically illustrates the makeup of the tread 6 for a tire of size 53/80R63, the said tread 6 being, according to an embodiment of the invention, made up of at least two radially superposed layers of blended elastomeric compound, a first layer of blended elastomeric compound forming a radially inner layer of the tread being made up of a first blended elastomeric compound forming a part extending at least in the region of the equatorial plane and of at least two axially outer parts formed of a second blended elastomeric compound and the radially outer layer of the tread being made up of a third blended elastomeric compound.

The voids ratio in the central part of the tread 6 is 3%. The voids ratio in the axially outer parts of the tread 6 is 29%.

According to an embodiment of the invention, the tread 6 is made up of a first layer 61 formed of a first blended elastomeric compound M1 forming a part 61 a extending at least into the region of the equatorial plane XX′ and of at least two axially outer parts 61 b formed of a second blended elastomeric compound M2.

The filled blended elastomeric compound M1 has a macro dispersion Z-value of 58 and a tan(δ)max value of 0.087.

The blended elastomeric compound M2 has a tan(δ)max value of 0.098.

The blended elastomeric compound M2 has a complex dynamic shear modulus G* 1% at 100° C. of 2.1.

The blended elastomeric compound M1 has a complex dynamic shear modulus G* 1% at 100° C. of 1.59.

The value of the complex dynamic shear modulus G* 1% at 100° C. of the blended elastomeric compound M2 is higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the blended elastomeric compound M1 by 32% and therefore, in accordance with the invention, by at least 10%.

The tread 6 comprises a radially outer second layer 62 that comes into contact with the ground and is made up of a third blended elastomeric compound M3.

The blended elastomeric compound M3 has a complex dynamic shear modulus G* 1% at 100° C. of 2.13.

The value of the complex dynamic shear modulus G* 1% at 100° C. of the blended elastomeric compound M3 is higher than that of the complex dynamic shear modulus G* 1% at 100° C. of the blended elastomeric compound M1 by 34% and, therefore, in accordance with the invention, by at least 10%.

The elongation at break of the blended elastomeric compound M3 in a tearability test at 100° C. is 320%.

The elongation at break of the blended elastomeric compound M1 in a tearability test at 100° C. is 275%.

The elongation at break of the blended elastomeric compound M2 in a tearability test at 100° C. is 238%.

The value of the elongation at break of the blended elastomeric compound M3 in a tearability test at 100° C. is higher than that of the elongation at break of the blended elastomeric compound M1 in a tearability test at 100° C. by 16% and, therefore, in accordance with the invention, by at least 10%.

The value of the elongation at break of the blended elastomeric compound M3 in a tearability test at 100° C. is higher than that of the elongation at break of the blended elastomeric compound M2 in a tearability test at 100° C. by 34% and, therefore, in accordance with the invention, by at least 10%.

The tread 6 also comprises a radially innermost additional layer 63 made up of a blended elastomeric compound M4 with a tan(δ)max value of 0.060.

The compounds M1, M2, M3 and M4 are described in the table below, together with a number of their properties.

Com- Com- Com- Com- pound M1 pound M2 pound M3 pound M4 NR 100 100 100 100 Black N134 35 40 Black N234 35 Black N347 34 Silica 170G 10 10 15 10 Anti-ozone wax C32 ST 1 1 1 Antioxidant (6PPD) 1.5 2.5 1.5 1 Silane on black 1 2 PEG (6000-20000) 1.67 2.5 Stearic acid 1 2 2 2 Accelerant CBS 1.7 1.4 1.7 1.35 Sulphur sol 2H 1.2 1.25 1.2 1.45 Zinc oxide 2.7 3 3 4.5 Z value 58 43 54 40 MSA300/MSA100 1.33 1.25 1.3 1.25 G*1% return 1.59 2.1 2.13 2 Elongation at break (% at 275 238 320 200 100° C.) tan(δ)_(max) 0.087 0.098 0.122 0.060

The thickness d₆₁ of the first layer 61 is 64 mm.

The thickness d₆₂ of the second layer 62 is 14 mm.

The thickness d₆₃ of the third layer 63 is 22 mm.

The ratio of the thickness d₆₂ of the second layer 62 to the thickness d₆₁ of the first layer 61 is 308% and therefore higher than 100%.

The ratio of the thickness d₆₃ of the additional layer 63 to the total thickness of the tread, namely the sum of the thicknesses (d₆₁+d₆₂+d₆₃) is 17% and therefore between 15 and 25%.

The thicknesses are measured on a meridian section of a tire in the new state, in the radial direction at the end of the radially outermost working layer.

Tests were carried out using vehicles fitted with the tires according to the invention in order to evaluate the wearing properties thereof.

These tests involve running tires fitted to the driven rear axle of a vehicle. The tires are inflated to a pressure of 7 bar and subjected to a load of 87.5 tonnes. The vehicles are driven along a track inclined at 14% successively uphill and downhill for a total duration of 1500 hours. The track is made up of stones ranging in size between 15 and 30 mm.

These tests are carried out with tires according to the depiction of FIG. 2, denoted P1.

These tires are compared against reference tires R1 which are the same size as the tires P1 and are fitted to the same vehicles. The tires R1 are tires which are known for this type of application.

The treads of the reference tires R1 comprise two radially superposed layers, the radially outer layer being made up of a first material A1 and the radially inner layer being made up of a material A2.

The thickness of the radially outer layer made up of the material A1 is 106 mm and the thickness of the radially inner layer made up of the material A2 is 22 mm.

The makeup and properties of these materials are described in the table below.

Compound A1 Compound A2 NR 100 100 Black N330 50 Black N347 34 Silica 170G 10 Anti-ozone wax C32 ST 1 Antioxidant (6PPD) 1.5 1 Silane on black 2 PEG (6000-20000) Stearic acid 2 2 Accelerant CBS 1.7 1.35 Sulphur sol 2H 0.6 1.45 Zinc oxide 2 4.5 Z value 48 40 MSA300/MSA100 1.28 1.25 G*1% return 2.01 2 Elongation at break (% at 230 200 100° C.) tan(δ)_(max) 0.135 0.060

The results obtained when running under the conditions described hereinabove demonstrated improvements in wear of between 15 and 20% with tires according to the invention as compared against the reference tires.

Further testing was carried out using vehicles fitted with the tires according to the invention in order to evaluate their properties in terms of resistance to attack.

The latter testing involved running the vehicles along a looped 500-metre track comprising a region 50 meters long made up of stones sized between 145 and 200 mm. The runs lasted for 500 hours at a speed of 15 km/h on the circuit outside of the stony region and 5 km/h over the said stony region.

The tires P1 according to an embodiment of the invention of size 53/80R63 are inflated to 7 bar and subjected to a load of 74 tonnes.

After running the tires are stripped and the number of cracks that reach the crown reinforcement counted. The number of cracks reaching the crown reinforcement is an indication of how well a tire resists attack.

As in the case of the first tests, the tires according to an embodiment of the invention P1 are compared against the reference tires R1 described hereinabove and fitted to the same vehicles.

The results obtained during these tests demonstrated improvements of the order of 20% in terms of resistance to attack of the tires according to embodiments of the invention as compared with the reference tires.

The results of these two types of testing show that the tires according to embodiments of the invention lead to a compromise between better wear performance and better resistance to attack. 

The invention claimed is:
 1. A tire with a radial carcass reinforcement, comprising: a crown reinforcement, a tread radially capping the crown reinforcement, connected to two beads by two sidewalls, the tread comprising at least two layers of blended elastomeric compounds that are radially superposed and have a voids ratio that is lower in the central part than at the axially outer parts, wherein a first layer of blended elastomeric compounds of the tread is made up of: a first blended elastomeric compound forming a part extending at least into the region of the equatorial plane, and at least two axially outer parts formed of a second blended elastomeric compound, wherein the first blended elastomeric compound has a macro dispersion Z-value higher than 55, wherein the first blended compound and the second blended compound have a maximum tan(δ) value, denoted tan(δ)max, at 100° C. of lower than 0.110, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the second blended elastomeric compound has a value at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. in MPa of the first blended elastomeric compound, wherein a second layer of the tread, radially on the outside of the said first layer, is made up of a third blended elastomeric compound of which the complex dynamic shear modulus G* 1% at 100° C. in MPa has a value at least 10% higher than that of the complex dynamic shear modulus G* 1% at 100° C. in MPa of the first blended elastomeric compound, and wherein the elongation at break of the third blended elastomeric compound in a tearability test at 100° C. has a value at least 10% higher than those of the elongations at break of the first and second blended elastomeric compounds in a tearability test at 100° C.
 2. The tire according to claim 1, wherein a ratio MSA300/MSA100 of the first blended elastomeric compound has a value at least 5% higher than that of a ratio MSA300/MSA100 of the second blended elastomeric compound.
 3. The tire according to claim 1, wherein the ratio MSA300/MSA100 of the first blended elastomeric compound has a value which differs by at most 5% from that of the ratio MSA300/MSA100 of the third blended elastomeric compound of the second layer.
 4. The tire according to claim 1, wherein the first blended elastomeric compound contains, by way of reinforcing filler, at least carbon black used at a content of between 10 and 70 phr, and wherein the carbon black has a BET specific surface area higher than 100 m²/g.
 5. The tire according to claim 1, wherein the first blended elastomeric compound contains, by way of a cut of carbon black, with a BET specific surface area higher than 100 m²/g, and of a white filler, wherein the reinforcing filler is used at a content of between 10 and 90 phr, and wherein the ratio of carbon black to white filler is higher than 2.7.
 6. The tire according to claim 1, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the second blended compound is higher than 1.6.
 7. The tire according to claim 1, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the first blended compound is higher than 1.4.
 8. The tire according to claim 1, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the third blended compound is higher than 1.6.
 9. The tire according to claim 1, wherein the thickness, measured in the radial direction at the end of what in a meridian section of the tire is the radially outermost working layer, of the second layer of blended elastomeric compound is greater than 15% of the thickness, measured in the radial direction at the end of what in a meridian section of the tire is the radially outermost working layer, of the first layer of blended elastomeric compound.
 10. The tire according to claim 1, wherein the tire further comprises an additional layer of a blended elastomeric compound in a radially innermost position of the tread, and wherein the blended elastomeric compound of the additional layer has a maximum tan(δ) value, denoted tan(δ)max, at 100° C. of lower than 0.80.
 11. The tire according to claim 6, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the second blended compound is 2.4 or lower.
 12. The tire according to claim 7, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the first blended compound is 2 or lower.
 13. The tire according to claim 8, wherein the complex dynamic shear modulus G* 1% at 100° C. in MPa of the third blended compound is 2.4 or lower. 